Difficulties of sex determination in education

14 years old, coming home from school, I was much more likely to question how no-one could recognise that Hannah Montana was Miley Cyrus in a blond wig than the wave-particle duality of the electron. For my LMA Manager-loving, running out of credit-hating teenage self, the electron was merely a negatively charged particle which orbited around the nucleus. This is what I was taught at school, and subsequently since entering teaching from a chronic fear of entering the working world or an inherent desire to tell people what to do, I teach my students. It is only as we progress in our education do we strip the simplicity away from such models and are provided with a more detailed, scientifically accurate analysis of the behaviour of electrons; at a time when our brains have (hopefully) matured and are capable of grasping more abstract ideas.

As a biology teacher, especially when teaching the inheritance of certain traits and characteristics, processes are ‘dumbed down’ to make them more graspable. For a very brief (and simplified) crash-course on genetics, DNA is in the form of chromosomes (long coiled up lengths of DNA) in which we have 46 (23 pairs) in our body cells, 23 we inherited from our mother and 23 from our father. In sperm and egg cells there are 23 so that when they fuse together in fertilisation it restores the number 46 (see diagram below). These chromosomes contain about 20-25,000 genes which code for proteins, molecules which have a wide range of functions such as making up collagen in your skin, keratin in your hair and nails, haemoglobin in your red blood cells, and enzymes which speed up all the reactions in your body. Conditions can arise when individuals have more or less chromosomes than the 23 pairs previously mentioned. The most commonly known one is Down Syndrome, where an individual has 3 copies (not 2) of chromosome 21. Between males and females there are 22 pairs of chromosomes which are the same, these are called the autosomes, but in school we teach that the 1 pair of chromosomes, the sex chromosomes, are the ones which differentiate the males from the females. Males have an X and a Y chromosome, whilst females have two X chromosomes.

We have 46 chromosomes in our cells. 23 pairs - one copy of each chromosome from our father and one from our mother. The chromosomes contain the genes which control our development.

After fertilisation, the fertilised egg (or zygote) divides to form an embryo, further developing into a foetus, before being born after the usual gestation period of ~9 months. All the muscle cells, nerve cells, bone cells, liver cells etc that make up the baby come from a single individual cell which has divided many times. In the embryo, the cells receive cues which cause some genes to be switched and some to be switched off. What genes are switched on and off will determine what type of specialised cell it will become - muscle, liver, skin, white blood cell etc. This process is called differentiation (see diagram below). So a cell which is destined to become a muscle cell will receive different cues to that which will become a kidney cell, and therefore switch different genes on and off. These cues also tell the cells in the embryo which sex organ to become.

Stem cells in the embryo receive different signals so become different cells in the body

Up until about 6 weeks both male and females have identical ‘bipotential’ gonad cells - these are the cells which are going to become specialised to form either the testes or the ovaries. Which pathway the cells will take is determined by a gene on the Y chromosome called SRY. If the cells have a Y chromosome, the SRY protein produced will switch the genes on which result in the development of testes. In the absence of this signal e.g. in individuals with two X chromosomes, the cells will differentiate into cells of the ovary. A similar pattern holds up when you look at the development of the male (sperm tube and prostate) and female (fallopian tube and uterus) internal plumbing. For the first 8 weeks of our foetal development we possess both female and male internal plumbing (ducts). If testes have developed after week 6 then they produce the hormone testosterone. It is this hormone which causes the male ducts to carry on developing whilst another signal causes the female ducts to break down. No signal results in the female ducts developing and the male ducts breaking down (see diagram below). Furthermore, the external genitalia in males and females come from the same structure, with testosterone signalling the development of the penis and scrotum, and in its absence the clitoris and labia develop (see diagram below). So pretty black and white?

Testosterone stimulates the male ducts to develop whilst another factor signals for the female ducts to break down. In the absence of signals the female ducts develop

Male and female external genitals develop from the same structure. Testosterone signals male development, no signal causes female development

Not quite. There are many conditions where this developmental pathway is not followed. One example being Complete Androgen Insensitivity Syndrome (CAIS) which I came across when reading Carole Hooven’s book, Testosterone. In this case a foetus’ cells contain the XY chromosome combination and as a result develop internal testes (due to the SRY gene on the Y chromosome). These testes release testosterone, however a mutation means the cells of the foetus do not respond to the testosterone. The signal which shuts down female duct development is still there but the foetus will develop female external genitalia. The cells act as if the testosterone wasn't there. As a result, the individual possesses a very visually female appearance including a vagina, clitoris etc. but will possess no uterus or ovaries, and instead have testes which will not have descended externally into a scrotum. (See link for video) Alternatively, de la Chapelle syndrome occurs when the SRY gene (the one needed for male sex organ development) is ‘copied and pasted’ onto the X chromosome, when a male is producing sperm. As a result, a fertilised egg produced from this sperm will have two XX chromosomes but develop as a male due to the presence of the SRY gene. These are just a couple examples of conditions in which there is no clear sex determination of XX being female and XY being male. Is this just another case of teaching simplistic scientific concepts which can be corrected and restructured at a higher level?

For me, not quite. Whilst there is no social and psychological damage to an electron, for simply describing it as a particle, or describing gravity as a force of an attraction between two objects rather than the warping of spacetime, cementing the idea that all XX people are females and XY males can create ideas, which once rooted, are much harder to overcome and change. Imagine being someone who suffered from CAIS, in which physiologically and externally you appear female, but then being told you are a male because of the combination of your chromosomes. My concern lies in when we teach these ideas early we may create stigmas against those individuals who don’t conform to that pattern which then have to be overcome, rather than addressing these variations right from the beginning. There are numerous psychological studies showing the difficulty in changing preconceived notions even in the presence of evidence opposing these ideas. For biological sex determination, the simplified, condensed teaching method in the syllabus should come with a caveat.